INTRODUCTION

The patient’s mouth is a challenging environment in which accurate impression is

made. Moisture control is probably one of the most important aspects of successful

impression making. Complete control of the environment of the operative site is essential

during restorative dental procedures. For the patients comfort and safety, and for the

operator's access and clear visibility, saliva, as well as water introduced during

instrumentation, must be removed from the mouth.

Control of the oral environment extends to the gingiva surrounding the teeth being

restored. The gingiva must be displaced to make a complete impression and sometimes even

to permit completion of the preparation and cementation of the restoration. Occasionally it is

necessary to permanently alter the contours of the gingival tissues around the teeth or of the

edentulous ridge to insure a better, longer-lasting result for the fixed restoration. Improper

manipulation of the impression material and tissue displacement can lead to permanent soft

tissue damage.

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FLUID CONTROL METHODS

During the preparation of teeth, it is necessary to remove large volumes of water

produced by a hand piece spray and to control the tongue to prevent accidental injury. Several

types of attachments can be used with low-volume (saliva ejector) or high volume vacuum

outlet to remove fluids. Some combine the functions of fluid removal with isolation1.

Rubber Dam

The rubber dam is the most effective of all isolation devices utilized in restorative

dentistry. In areas where only supragingival margins are present, moisture control with

appropriate method. Its use is valuable in

1. The removal of old restoration or excavation of caries when exposure of the pulp is a

possibility.

2. It also provides excellent isolation and access when a pin-retained amalgam or

composite resin core is required before a cast restoration can be fabricated.

3. Teeth with old or questionable endodontic treatments should be isolated in this

manner for dowel-core preparation, pattern fabrication, and cementation.

4. It can be readily used for making impressions and cementing the same types of

restorations.

When used with elastomeric impression materials, the dam must be lubricated and

the clamp must be removed or avoided. It should not be used with polyvinyl siloxane

impression material, because the rubber dam will inhibit its polymerization.

Absorbent cotton rolls

Absorbent cotton rolls must be placed at the source of the saliva. In the maxillary

arch, placing a single cotton roll in the vestibule immediately buccal to the preparation and a

saliva evacuator in the opposing lingual sulcus is usually efficient. In the maxillary second or

third molar, multiple cotton rolls must be immediately placed buccal to the preparation and

slightly anterior to block off the parotid duct, which opens just anterior to the maxillary first

molar. When making mandibular impression, placing additional cotton rolls to block off the

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sublingual and submandibular salivary ducts is usually necessary. Rolls on the buccal and

lingual sides of the prepared teeth will help with soft tissue retraction – the cotton on the

lingual side displaces the tongue medially and the cotton on buccal side displaces the cheek

laterally.

High-Volume Vacuum

A high-volume suction tip is extremely useful during the preparation phase and is

most effectively utilized with an assistant. Three commonly used vacuum attachments

include saliva ejector, svedopter and vacuum tip.

Saliva Ejector and vacuum tip

It is most useful as an adjunct to high-volume evacuation, but it can be used alone for

the maxillary arch. The saliva ejector is placed in the corners of the mouth, opposite the

quadrant being operated, and the patient's head is turned toward it. It can also be used very

effectively on the maxillary arch for impressions and cementation simply by adding cotton

rolls in the vestibule facing to the tooth being isolated. It can be used on the mandibular arch

while a cotton roll holder positions cotton rolls facial and lingual to the teeth. Tongue control

and fluid removal in this application may be less than ideal, however.

Svedopter

It consists of a metal saliva ejector with attached tongue deflector is excellent. It is

especially useful in mandibular arch. It can be used without cotton rolls during the

preparation phase, with a mouth mirror as a lip retractor. By adding facial and lingual cotton

rolls, excellent tongue control and isolation is provided for impression making or

cementation.

The Svedopter is most effective when it is used with the patient in a nearly upright

position, water and other fluids collect on the floor of the mouth, where they are pulled off by

the vacuum. If the patient is, in a supine position, the throat and back of the mouth must fill

with fluid before it reaches the level of the evacuation device. For better positioning, the

anterior part of the Svedopter should be placed in the incisor region, with the tubing under the

patient's arm. This provides the security of having the tubing firmly under the patient's

control.

Drawbacks:

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1. Access to the lingual surfaces of the mandibular teeth is limited.

2. It causes bruising of tender soft tissues in the floor of the mouth.

3. The presence of mandibular tori usually precludes its use.

4. Selection of an oversized reflector should be avoided, since it could cut into the palate

above or trigger the gag reflex.

Antisialagogues

For the patient who salivates excessively drugs can be used to control salivary flow.

Methantheline bromide (Bantine) and propantheline bromide (Pro-Banthine), have been used

for this purpose. They are gastrointestinal anticholinergics that act on the smooth muscles of

the gastrointestinal, urinary, and biliary tracts, producing a dry mouth as a side effect. Usually

one 50mg tablet of Banthine or 15-mg tablet of pro-banthine taken one hour before the

appointment will provide the necessary control.

Side effects :

The patient may experience drowsiness and blurred vision.

Unfortunately, these substances also produce an unpleasant, bitter aftertaste.

Contraindications :

History of hypersensitivity the drugs,

Eye problems such as glaucoma,,

Asthma

Obstructive conditions of the gastrointestinal or urinary tracts,

Congestive heart failure

They should not be used by lactating females.

Another drug that has been shown to be effective as an antisialalogue is clonidine

hydrochloride. Clonidine hydrochloride is an antihypertensive agent, and it should be used

cautiously in patients who are receiving other antihypertensive medication.. The dose of 0.2

mg should be administered an hour before the appointment.

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GINGIVAL RETRACTION

Gingival retraction is a process of exposing margins when making impressions of

prepared tooth3.

Other names : Tissue dilatation, tissue retraction and tissue displacement .

Advantages of gingival retraction:

1. Duplicating subgingival margins in the impression.

2. Recording the unprepared tooth surface to aid in evaluating the marginal finish line

accurately

3. Helps in accuracy of wax pattern fabrication and location of finish line.

4. Final restoration tends to have better marginal adaptation. To alter the contour of the

gingival tissue around the teeth or edentulous ridge.

Classification of gingival retraction:

Mechanical: retraction by mechanical methods.

Chemicomechanical: retraction with a cord impregnated with a chemical for hemostasis.

Surgical: retraction by removal of gingival tissue from the sulcus with dental

electrosurgery.The surgical techniques can be further broken down into rotary curettage and

electrosurgery.

MECHANICAL

Physically displacing the gingiva was one of the first methods used for insuring

adequate reproduction of the preparation finish line.

A copper band or tube can serve as a means of carrying the impression material as

well as a mechanism for displacing the gingiva to insure that the gingival finish line is

captured in the impression.

One end of the tube is festooned, or trimmed, to follow the profile of the gingival

finish line, which, in turn, often follows the contours of the free gingival margin. The tube is

filled with modeling compound, and then it is seated carefully in place along the path of

insertion of the tooth preparation. It has been used with impression compound and

elastomeric materials. If the impression is made with an elastomeric material, the die can be

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formed of stone or electroplated metal; if the impression is compound, the die can be made of

amalgam or electroplated metal2.

Drawbacks:

Causes incisional injuries of gingival tissues,

Recession following their use ranges from 0.1 mm in healthy adolescents to

0.3 mm in a general clinic population.

The likelihood of capturing all of the finish lines in one impression decreases

as the number of prepared teeth increases.

A rubber dam also can accomplish the exposure of the finish line needed. Generally

it is used when a limited number of teeth in one quadrant are being restored and in situations

in which preparations do not have to be extended very far sublingually. It can be used with

modified trays if the bow and wings of the clamp are blocked out. A rubber dam should not

be used with polyvinyl siloxane impression material, because the rubber inhibits its

polymerization.

String or fibres of different types have been advocated for placing in the gingival

sulcus. They include Plain cotton cord, unwaxed floss, cotton thread, and 2/0 untreated

surgical silk. They are used for sulcus enlargement, physically pushing' away the gingiva

from the finish line. Unfortunately, its effectiveness is limited because the use of pressure

alone often will not control sulcular hemorrhage4.

A technique has been described where a temporary acrylic coping is construted. The

inside is relieved by approximately 1mm, and adhesive is applied. It is then filled with

elastomeric impression material and reseated. The tissue is displaced when the impression

material is mechanically forced into the sulcus. A complete arch impression is subsequently

made over the coping , so that it becomes an integral part of the complete arch impression.

Alternatively a temporary metal crown can be fabricated which is lined with an excess

of temporary stopping material. It is left in place until the next appointment and final

impression made.

Chemicomechanical (retraction cord)

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By combining chemical action with, pressure packing, enlargement of the gingival

sulcus as well as control of fluids seeping from the walls of the gingival sulcus is more

readily accomplished.

Various drugs used include:

0.1% and 8% racemic epinephrine

100% alum solution ( potassium aluminium sulphate)

5% and 25% aluminium chloride solution

Ferric subsulphate solution( Monsel’s solution)

45% negatol solution

The three criteria for a gingival retraction material are

(1) effectiveness in gingival displacement and hemostasis,

(2) absence of irreversible damage to the gingiva, and

(3) paucity of untoward systemic effects.

Epinephrine produces hemostasis, and it causes local vasoconstriction, which in turn

results in transitory gingival shrinkage.

Side effects :

Elevation of blood pressure and

Increased heart rate.

Contraindication :

1. Patients with cardiovascular disease, hypertension diabetes, hyperthyroidism, or a

known hypersensitivity to epinephrine.

2. Epinephrine also should not be used on patients taking Rauwolfia compounds,

ganglionic blockers, or epinephrine-potentiating drugs.

3. Patients taking monoamine oxidase inhibitors for depression should not receive

epinephrine.

Patients without the aforementioned contraindications can also exhibit "epinephrine

syndrome" (tachycardia, rapid respiration, elevated blood pressure, anxiety, and

postoperative depression). The symptom appears either after after the cord has been in for

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few minutes or shortly after removal. It has been recommended that 0.1% epinephrine should

be used rather than the 8% solution.

The amount of epinephrine absorbed is highly variable, depending on the degree of

exposure of the vascular bed, as well as the time of contact and the amount of medication in

the cord. The amount of epinephrine lost (and presumed absorbed) from 2.5 cm of typical

retraction cord during 5 to 15 minutes in the gingival sulcus is 71 µg. This amount is slightly

less than that obtained from receiving the injection of four carpules of local anesthetic

containing a 1:100,000 concentration of epinephrine. It also is approximately one-third the

maximum dose of-0.2 mg (200 (µg) for a healthy adult and nearly twice the recommended

amount of 0.04 mg (40 µg) for a cardiac patient.

Alum (aluminum potassium sulfate)

100% concentration has been shown to be only slightly less effective in

shrinking gingival tissues than epinephrine.

It shows good tissue recovery.

A 0.1 mm of permanent crestal bone loss is usual.

The recommended time of use is as long as 20 min without adverse effects5.

Aluminium chloride

It is one of the most commonly used chemicals in the concentrations of 5%

and 25%.

Solutions stronger than 10% can produce local tissue destruction.

It is found to be most effective in controlling bleeding and in displacing tissue

with no resultant tissue damage.

Ferric subsulphate

It is slightly more effective in tissue displacement than epinephrine.

The tissue recovery is good.

The recommended time of use is 3min.

The ferric or ferrous salts should not be used as they are corrosive and

injurious to soft tissues and enamel and because they stain teeth. This is

attributed to high acidity(72%) of the solution.

Ferric sulphate (13.3%)

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It is not compatible with epinephrine because it forms a massive blue

precipitate

It cogulates blood so quickly that it must be kept directly against the cut tissue.

If not , the ferric sulphate becomes tied up with the extravasated blood and

floats away, leaving a bleeding surface.

The recommended use time is 1to 3 min.

The resulting tissue displacement is maintained for atleast 30 min, so that

repacking is seldom necessary for multiple impressions.

Zinc chloride (bitartrate)

It has been used in 8% and 40% solutions.

Gingival displacement effectiveness of 8% solution is similar to that of

epinephrine. 40% solution is so caustic and is a chemical cautery agent.

Its use is not recommended as gingival displacement as it causes permanent

tissue injury.

Tannic acid ( 20% and 100%) is less effective than epinephrine but shows good

tissue recovery. The recommended use is for 10min.

Negatol is a 45% condensation product of meta cresol sulphonic acid and

formaldehyde. Tissue recovery is poor. It is highly acidic and decalcifies teeth. It is classified

as a chemical cautery agent and is not recommended for gingival displacement.

Procedure for gingival retraction :

The operating area must be dry. An evacuating-device is placed in the mouth,

and the quadrant containing the prepared tooth is isolated with cotton rolls.

The retraction cord is drawn from the dispenser bottle with sterile cotton

pliers, and a piece approximately 5.0 cm (2.0 inches) long is cut off.

The retraction cord should be moistened by dipping it in buffered 25%

aluminum chloride solution in a dappen dish. If there is slight hemorrhage in

the gingival crevice, it can be controlled by the use of a hemostatic agent, such

as Hemodent liquid (aluminum chloride)

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In any event, the cord must be slightly moist before it is removed from the

sulcus. Removing dry cord from the gingival crevice can cause injury to

delicate epithelial lining.

Form the cord into a "U" and loop it around the prepared tooth. Hold the cord

between the thumb and forefinger, and apply slight tension in an apical

direction.

Gently slip the cord between the tooth and the gingiva in the mesial

interproximal area with a Fischer packing instrument or a DE plastic

instrument IPPA.

Proceed to the lingual surface and begin working from the mesiolingual corner

around to the distolingual corner. The tip of the instrument should be inclined

slightly toward the area where the cord has already been placed; ie, the mesial.

Continue packing the cord around the facial surface, overlapping the cord in

the mesial interproximal area. The overlap must always occur in the proximal

area, where the bulk of tissue will tolerate the extra bulk of cord.

Pack all but the last 2.0 or 3.0 mm of cord. This tag is left protruding so that it

can be grasped for easy removal.

After 10 minutes, remove the cord slowly to avoid bleeding. Inject impression

material only if the sulcus remains clean and dry.

Electrocoagulation and ferric sulfate are sometimes effective in stopping

persistent bleeding.

Techniques of gingival displacement

The single cord technique :

It is indicated when making impressions of one to three teeth with healthy gingival

tissues. It is simple and most common method. The procedure described above is that of

single cord technique.

The double cord technique :

It is used when making impressions of multiple prepared teeth and when making

impressions when tissue health is compromised.A small diameter cord (2/0 surgical silk) is

placed in the sulcus. This cord should be lelf in the sulcus during impression making . A

second cord, soaked in the hemostatic agent is placed above the small-diameter cord. The

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diameter of the second cord should be the largest diameter that can be readily placed in the

sulcus.after 8-10 min second cord is removed. The preparations are dried and the impression

is made with the primary cord in place.

Infusion technique :

After preparation of the cervical margins in an intra-crevicular position, hemorrhage

is controlled using a specifically designed dentofusor with a ferric sulfate medicament. 20%

ferric sulfate is preferred. The infusor is used with a burnishing motion in the sulcus and is

carried circumferentially 3600 around the sulcus. The medicament is extruded as the

instrument is manipulated around the sulcus. Then a knitted retraction cord soaked in ferric

sulfate is packed into the sulcus for 1-3 min. It is then removed and the sulcus is rinsed with

water and impression made.

The every other tooth technique :

During anterior tooth preparations, when roots are in close proximity , placing

retraction cord simultaneously around all the teeth results in strangulation of gingival papillae

and loss of papilla.

Retraction cord is placed around the most distal prepared tooth. No cord is placed

around the prepared tooth anterior to this tooth. Retraction procedures are completed in

alternate tooth. The impression is made. Then gingival displacement is done on the remaining

prepared teeth and the second impression made. A subsequent pick up impression allows

fabrication of a master cast with dies for all individual prepared teeth.

Recent technique :

Expasyl ( Pierrre Rolland ,1999) is a new material available in chemico-mechanical

technique for sulcus opening and hemostasis. It saves considerable amount of time and

enhances comfort of both practioner and patient. It contains white clay to ensure consistency

of paste and its mechanical action, while aluminium chloride enhances the hemostatic action.

This paste is injected into the sulcus , exerting a stable non-damaging pressure of 0.1N/mm.

when it is left in place it is sufficient to obtain a sulcus opening of 0.5mm for 2min.

Rotary Curettage

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Rotary curettage is a "troughing" technique, the purpose of which is to produce

limited removal of epithelial tissue in the sulcus while a chamfer finish line is being created

in tooth structure. The technique which also has been called gingettage is used with the

subgingival placement of restoration margins1.

It differs from periodontal curettage in that it is used to debride diseased tissue from

the sulcus to allow reepithelialization and healing.

Rotary curettage however, must be done only on healthy, inflammation - free tissue to

avoid the tissue shrinkage that occurs when diseased tissue heals.

The concept of using rotary curettage was described by Amsterdam in 1954. The

technique described here was developed by Hansing and subsequently enlarged upon by

Ingraham.

The gingival should satisfy the certain factors :

Absence of bleeding upon probing,

Sulcus depth less than 3 mm, and

Presence of adequate keratinized gingiva.

In conjunction with axial reduction, a shoulder finish line is prepared at the level of

the gingival crest with a flat-end "tapered diamond. Then a torpedo-nosed diamond of 150 to

180 grit is used to extend the finish line apically, one-half to two-thirds the depth of the

sulcus, converting the finish line to a chamfer. A generous water spray is used while

preparing the finish line and curetting the adjacent gingiva. Cord impregnated with aluminum

chloride or alum is gently placed to control hemorrhage. The cord is removed after 4 to 8

minutes, and the sulcus is thoroughly irrigated with water. This technique is well suited for

use with reversible hydrocolloid.

Drawbacks :

Poor tactile sensation

Potential for destruction of periodontium if used incorrectly.

This method is best used only by experienced dentists.

Electrosurgery/ Surgical diathermy

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If the gingiva surrounding the tooth consists of areas of inflammation and

granulation tissue due to overhangs on previous restorations or by the caries itself ,it may

have been necessary to place the finish line of the preparation so near the epithelial

attachment . In these cases, it may be necessary to use some means other than cord

impregnated with chemicals to gain access and stop minor bleeding.

The use of electrosurgery has been recommended for enlargement of the gingival

sulcus and control of hemorrhage to facilitate impression making.

Credit for being the direct progenitor of electrosurgery is generally given to

Arsonval.An electrosurgery unit is a high-frequency oscillator or radio transmitter that uses

either a vacuum tube or a transistor to deliver a high-frequency electrical current of at least

1.0 MHz (one million cycles per second).

Current flows from a small cutting electrode that produces a high current density and

a rapid temperature rise at its point of contact with the tissue. The cells directly adjacent to

the electrode are destroyed by this temperature increase. The current concentrates at points

and sharp bends. Cutting electrodes are designed to take advantage of this property so they

will have maximum effectiveness. The circuit is completed by contact between the patient

and a ground electrode that will not generate heat in the tissue because its large surface area

produces a low current density.

Types of Current. There are different forms of current. They are significant because each

produces a different tissue response.

The unrectified, damped current is characterized by recurring peaks of power that

rapidly diminish. It gives rise to intense dehydration and necrosis. It causes considerable

coagulation, and healing is slow and painful. Sometimes referred to as the Oudin or Telsa

current. It is not used in dental electrosurgery today.

A partially rectified, damped (half-wave modulated) current produces a wave

form with a damping in the second half of each cycle. The damping effect produces good

coagulation and hemostasis, but tissue destruction is considerable and healing is slow.

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A better current for enlargement of the gingival sulcus is found in the fully rectified

(full-wave modulated) current that produces a continuous flow of energy. Cutting

characteristics are good and there is some hemostasis.

The fully rectified, filtered (filtered} is a continuous wave that produces excellent

cutting. The continuous wave produces less injury to the tissue than does a modulated wave.

Contraindications.

Patients with cardiac pacemakers.

Because it can produce sparks in use, electrosurgery should not be used in the

presence of flammable agents.

Electrosurgery Technique

Before an electrosurgical procedure is done, verify that anesthesia is profound

and reinforce it if necessary.

Check the equipment to make sure all the connections are solid.

Be especially certain that the cutting electrode is seated completely in the

handpiece.

Proper use of electrosurgery requires that the cutting electrode be applied with

very light pressure and quick, deft strokes.

The electrode is being guided, and not pushed, through the tissue.

To prevent lateral penetration of heat into the tissues with subsequent injury,

the electrode should move at a speed of no less than 7 mm per second.

If it is necessary to retrace the path of a previous cut, 8 to 10 seconds should

be allowed to elapse before repeating the stroke. This will minimize the

buildup of lateral heat that could disrupt normal healing.

As the electrode passes through the tissue, it should do so smoothly without

dragging or charring the tissue.

Moist tissue will cut best.

A high-volume vacuum tip should be kept immediately adjacent to the cutting

electrode at all times to draw off the unpleasant odors that are generated.

A wooden tongue depressor or plastic-handled mirror should be used rather

than the metal-backed mouth mirror that would customarily be employed.

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This technique is used for

Gingival Sulcus Enlargement

Removal of an Edentulous Cuff and

Crown Lengthening

Gingival Sulcus Enlargement

A small, straight or J-shaped electrode is selected. It is used with the wire parallel

with the long axis of the tooth so that tissue is removed from the inner wall of the sulcus. If

the electrode is maintained in this direction, the loss of gingival height will be about 0.1 mm.

A whole tooth should be encompassed in four separate motions: facial, mesial,

lingual, and distal at a speed of no less than 7 mm per second. If a second pass is necessary

in any one area, wait 8 to 10 seconds before repeating that stroke. This will minimize the

production of lateral heat. Clean tissue debris off the electrode tip after each stroke. Use a

cotton pellet dipped in hydrogen peroxide to clean debris from the sulcus. Better results are

usually obtained if retraction cord is loosely packed in the enlarged sulcus before the

impression is made.

Removal of an Edentulous Cuff

The remnants of the interdental papilla adjacent to an edentulous space will form a

roll or cuff that will make it difficult to fabricate a pontic with cleanable embrasures and

strong connectors. If there are cuffs, they should be removed. A large loop electrode is used

for planning away the large roll of tissue.

Crown Lengthening

There are circumstances in which it may be desirable the have a longer clinical crown

on a tooth than is present. If there is a sufficiently wide band of attached gingiva surrounding

the tooth, this can be accomplished with a gingivectomy using a diamond electrode. It is

frequently necessary to do a second series of cuts to produce a bevel around the first. This

will produce a better tissue contour. When surgery leaves an extensive postoperative wound,

as in this case, it is necessary to place a periodontal dressing, which should be changed in

about 7 days.

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TECHNICAL CONSIDERATIONS AND MANIPULATION OF

IMPRESSION MATERIALS

An impression is an imprint or negative likeness. From this negative form of the teeth

and surrounding structures, a positive reproduction, or cast, is made.

Requirements of an impression for cast restoration:

1. It should exactly duplicate the prepared tooth.

2. It should allow the dentist and technician to be certain of the location and

configuration of the finish line.

3. The teeth and tissue adjacent to the prepared tooth must be accurately reproduced

to permit proper articulation of the cast and contouring of the restoration.

4. It must be free of bubbles, especially in the area of the finish line and occlusal

surfaces of the other teeth in the arch.

Impression Materials:

There are several types of impression materials that are accurate enough to be

considered for cast restorations. The materials consist of

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1. Reversible Hydrocolloid,

2. Polysulfide,

3. Condensation Silicone,

4. Polyvinyl Siloxane, And

5. Polyether.

REVERSIBLE HYDROCOLLOID :

Other names : agar hydrocolloid or simply hydrocolloid.

It was originally derived from a natural product of kelp2.

Hydrocolloid technique :

Hydrocolloids are supplied as a gel in a sealed tube or clear plastic casing, which can

be heated in water and softened. The material is available in different viscosities. The low

viscosity material is syringed around the teeth and the high viscosity material is loaded in the

tray and inserted over the teeth. Cool water is circulated over the tray to produce gelation.

Preparation of the material :

The material is supplied in two forms:

Small sticks or cartridges are available for use in the syringes. It can be injected in

and around the prepared teeth.

Metal or plastic tubes are available for use as tray material . water cooled trays are

used to carry the hydrocolloid into the mouth to form the bulk of the impression.

The only difference between the tray and syringe material is its color and greater

fluidity in the syringe material.

There are three steps in preparation of the impression material

1. Liquefying the material – to reverse the gel to sol state. It is done by

placing the material in the liquefying bath of the conditioner. Tubes and

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cartridges should be boiled for 10min. If the material is reliquefied it should

be placed for 12min to reach the boiling temperature of 2120F (1000C).

2. Storage of the material – A storage temperature of 660C (1510F) to 680C

(1550F) is usually ideal. The material can be stored for 5 days.

3. Tempering or conditioning – loaded impression trays are tempered in this

bath at 1100F (430C) to 1150F (460C) for 5 to 10 min immediately before it

is placed in the mouth. The purpose of conditioning is to increase the

viscosity of the hydrocolloid so that it will not flow out of the tray.

Making the impression

After the tray material has been conditioned, the prepared teeth are covered with

hydrocolloid sol using the syringe material. Inject hydrocolloid from the syringe into the

sulcus, starting in an interproximal area first. Hold the tip above the mouth of the crevice,

taking care not to drag the tip along the gingiva. Proceed smoothly around the entire

circumference of the preparation, pushing impression material before the tip.

After the prepared teeth have been covered with the syringe material, the tray material

is properly conditioned and is ready to be placed immediately in the mouth to form the bulk

of the impression. The tray is seated under passive pressure. The sol in the tooth bonds to the

hydrocolloid in the tray to form a homogenous impression.

Gelation is accomplished by circulating cool water, approximately 180C to 210C (640F

to 700F) through the tray for not less than 5 min. Care must be exercised to prevent any

movement of the tray during the gelation time. After gelation the impression is withdrawn in

one piece. The temperature at which the hydrocolloid material sets is known as gelation

temperature . It ranges between 360C( 970F) and 420C( 1080F).

The accuracy can be improved if the material has as much bulk as possible( low

surface area/ volume ratio).This contrasts with elastomeric impression materials whose

accuracy is improved by minimizing the bulk, because stress produced during removal are

reduced. Therefore, an additional advantage of reversible hydrocolloid is that a custom

impression tray is not required.

Other techniques:

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An alternative way of applying the syringe material is called the wet field technique.

The prepared teeth are bathed in warm water, and syringe material is deposited in generous

quantities only on the occlusal surfaces of the teeth. The relatively viscous tray material is

counted on to force the lighter-bodied syringe material into the sulcus as the tray is seated.

This technique should be used only on preparations that do not contain internal features, such

as grooves, boxes, or isthmuses.

A technique combining reversible and irreversible hydrocolloid was described in 1951

and a modified reversible hydrocolloid that could bond to alginate was introduced in late

1970 s. This system is also known as laminate technique. It has the advantage of requiring

less complicated instruments for liquefaction and storage. The alginate in the tray, mixed

with water at 700F, cools the reversible hydrocolloid that has been injected around the

prepared tooth, causing it to solidify. This eliminates the need for water jacketed trays and

tubing. The impression should be ready to remove from the mouth in 3 min, making it faster

than other elastic materials. It is also less expensive.

ELASTOMERIC IMPRESSION MATERIALS

These material are synthetic rubbers that are known technically as nonaqueous

elastomers to differentiate them from hydrocolloids. There are four types of elastomeric

impression materials available for crown and fixed partial denture impressions:

Polysulphide

Condensation silicone

Addition silicone

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Polyether

In addition to four chemical types, elastomeric materials are available in different

viscosities. They are:

Very high viscosity (putty)

High viscosity (heavy body or tray material)

Medium viscosity (regular material)

Low viscosity (light body or syringe)

Before going to the impression making with elastomeric impression material , a

knowledge about the impression techniques, mixing systems, and a basic knowledge about

each impression material is essential.

IMPRESSION TECHNIQUES FOR ELASTOMERIC IMPRESSION

MATERIAL

Three common methods for making impressions for fixed restorations are

Simultaneous , dual viscosity technique

A single viscosity or monophase technique and

A putty wash technique.

Simultaneous , dual viscosity technique is one in which low-consistency material is

injected with a syringe into the critical areas and the high-consistency material is mixed and

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placed in an impression tray. After injecting the low- viscosity material, the tray containing

the higher viscosity material is placed in the mouth. In this manner , the more viscous

material forces the lower-viscous material into the fine aspects of the areas of interest.

Because they both are mixed at the same time , the materials join and bond, and set together.

After the material has set , the tray and the impression are removed.

In the single –viscosity or monophase technique, impressions are often taken with a

medium – viscosity impression material. Addition silicone and polyether materials are well

suited for this technique because both have a capacity for shear thinning. When a medium

viscosity material is forced through an impression syringe, the viscosity is reduced, where as

the viscosity of the same material residing in tray is unaffected. In this manner, such

materials can be used for syringing and for trays.

The putty wash techniques

There are three approaches to putty/ wash impressions.

1. An excellent technique is to use the putty material to fabricate a custom tray

in the same manner as acrylic material. One layer od base plate wax is

placed over the diagnostic cast as a spacer. Wax is remover from the

nonfunctioning cusp to provide occlusal stop. Then a putty impression is

made in a stock tray and a putty custom tray results.

2. A preliminary impression is taken in high or putty consistency material

before tooth preparation is made. Space is provided for a low viscosity

material by plastic sheets, and after preparation a low consistency material

is syringed into the area and the preliminary impression is reinserted. The

low and high consistency materials bond, and after the low consistency

material sets, the impression is removed. This procedure is sometimes

called a wash technique.

3. Simultaneous or squash technique : A stock tray is loaded with putty

material, and the syringe material is injected around the prepared tooth. The

tray containing the putty is squashed over the syringe material, and the

impression is made with the putty material and the syringe material setting

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simultaneously. This approach is unacceptable as it is impossible to control

the thickness of impression material and excess bulk is used.

Other recent techniques

Dual arch impression technique :

When one or two posterior teeth are prepared for indirect restorations, it is

often advantageous to consider using dual arch or double bite impression technique.

This technique captures the prepared teeth , opposing arch, and the occlusal

articulation in maximum intercuspation simultaneously. This technique should be

used for a maximum of two prepared teeth. There should be unprepared stops anterior

and posterior to the prepared teeth. It should be used only in patients who have

existing anterior guidance. If not non working side interferences may be introduced in

the new restorations.

The segmental impression technique

This technique offers a procedure that predictably permits making a successful

impression with multiple prepared teeth. This technique can be used with any material

but is optimally used auto-mix poly vinyl siloxane. With this technique , the arch to

be impressed is broken down into easily managed segments as two prepared teeth per

segment. Individual custom trays are fabricated on the diagnostic cast for each

segment. All of the individual trays must be able to seat simultaneously on the cast.

After gingival displacement procedures are completed, low viscosity material

is mixed and loaded in syringe and one of the segmental trays. Retraction cord is

removed from one segment and the syringe material is injected around the preparation

and the tray seated. It is allowed and set and should not be removed. Same way is

done for all the segments and an over impression is made in a stock tray. This is

useful in extensive cases and when moisture control is difficult.

MIXING SYSTEMS

Three types of systems are available to mix the catalyst and base thoroughly before

making impression.

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Hand mixing

Static automixing and

Dynamic mechanical mixing .

Hand mixing :

Equal length of base and catalyst are dispensed on a paper pad.

Initial mixing is done with a circular motion and the final mixing is done with broad

strikes of spatula to produce a mix free from streaks.

Mixing should be done within 45 sec.

When the catalyst is supplied as a liquid, a specified number of drops per unit length

are mixed similar to that of two paste systems.

All four types are available for mixing in this fashion.

In case of putty system, offered with both addition and condensation silicone, scoops

are supplied by the manufacturer for dispensing. The putties are kneeded with fingers until

free from streaks.

Automixing system:

The base and catalyst are in separate cylinders of the plastic catridge. The catridge is

placed in a mixing gun containing two plungers that are advanced by a ratchet mechanism to

extrude equal amounts of base and catalyst. The base and catalyst are forced through the

mixing tip containing internal spiral resulting in a uniform mix at the tip end. All

consistencies except putty can be used with this system. Addition silicones, condensation

silicones and polyethers are available with this means of mixing.

Dynamic mechanical mixer :

The catalyst and the base are supplied in large plastic bags housed in a cartridge,

which is inserted into the top of the mixing machine. A new, plastic mixing tip is placed on

the front of the machine, and when the button is depressed, it results in opening of the plastic

bags and therefore forcing the material into the dynamic mixing tip. This mixing tip differs

from automixing in that the internal spiral is motor driven so it rotates. Thus mixing is

accomplished by this rotation plus forward motion of the material through the spiral.

Polyether and addition silicone are available for mixing with system.

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POLYSULPHIDE POLYMER:

Other names : vulcanizing impression material, mercaptan, Thiokol, Rubber base.

It is supplied in two tubes.

Base: Contains liquid polysulphide polymer mixed with an inert filler.

Accelerator: Lead dioxide mixed with sulfur and oil as an oxidation initiator on terminal

thiol groups or polymer.

The reaction is condensation polymerization type with water as the by-product .

Generally a double-mix technique is used with a heavy-bodied tray material and less

viscous syringe material. These polymerize simultaneously, forming a chemical bond.

Advantages :

The high tear resistance and enhanced elastic property.

It is radio opaque .

Disadvantages :

Not well tolerated by patients because of its unpleasant sulphide odor.

Long setting time in the mouth (about 10min).

Stability only fair as water evaporates from the surface of the impression

resulting in distortion.

The unpolymerized product is sticky stains cloth permanently. Alternatively,

copper hydroxide is commonly used.

CONDENSATION SILICONE:

Other names : Room temperature vulcanization silicone.

Base: Liquid silicone polymer with terminal hydroxyl groups, mixed with inert fillers.

Reactor : Viscous liquid, consists of crosslinking agent, ethyl silicate and tin octoate as

activator.

Reaction occurs between terminal hydroxyl groups on the polymer and orthosilicate.

Ethyl alcohol is the by-product of setting reaction.

Advantages :

It is essentially odourless.

Adequate working and setting time in the mouth( 6-8min).

No staining

Disadvantages:

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Poor dimensional stability.

Poor shelf life.

Impression should be poured immediately.

poor wetting characteristics ( hydrophobic)

Impression pouring without trapping air bubbles is also difficult.

ADDITION SILICONE :

Other names :poly vinyl siloxane, vinyl poly siloxane and vinyl silicone.

Supplied as two pastes

1. Silicone with terminal silane hydrogen groups and inert filler.

2. Silicone with terminal vinyl groups, chloroplatinic acid catalyst and a filler .

Upon mixing equal quantities of two, there is addition of silane hydrogen across vinyl

double bond with no by-products.

It is generally used as two-viscosity system, although monophase formulations are

also available. It is easier to trap air bubbles when using monophase. Pouring should be

delayed with some earlier products due to gas release from the impression. Newer products

contain scavengers that prevent the escape of gas at the polymer-cast interface. Additional

silicone containing scavenger material can be poured immediately.

Advantage:

No by-product

Exceptionally stable material

Can be poured even one week after removal from mouth

Disadvantage

Setting is inhibited by wearing latex gloves.

It is hydrophobic

POLYETHER:

It is a co-polymer of 1,2 epoxy ethane and tetrahydrofuran that is reacted with an α,β

unsaturated acid to produce esterification of terminal hydroxyl groups.

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Advantages :

No volatile by-product is formed.

Excellent dimensional stability.

It has short setting time in the mouth ( about 5min) .

Disadvantages

The set material is stiff which causes problems when separating a stone cast from the

impression.

Polysulphide is stable only if stored dry, because it will absorb moisture and undergo

significant dimensional change.

Isolated cases of allergic hypersensitivity have been reported.

CUSTOM RESIN TRAYS

Custom resin trays have been utilized in elastomeric impression techniques to obtain

an impression of uniform thickness around 2-3mm. The custom tray must be rigid, and it

should have stops on the occlusal surfaces of the teeth to orient the tray properly when it is

seated in the mouth. The impression material must adhere firmly to the tray. This is achieved

with a rubber adhesive packaged with the impression material.

Tray Preparation

Heat a sheet of baseplate wax in a flame until it is softened. Place it on the diagnostic

cast of the arch to be restored. Adapt it to the cast and trim any excess that extends more than

2 to 3 mm beyond the necks of the teeth. The wax will form a spacer for the impression

material. A horseshoe-shaped form is used for both arches, with no palatal coverage on the

maxillary arch.

Cut a 3 x 3-mm hole through the wax over posterior teeth on both sides of the arch

and in the incisor area. The tray resin will touch the teeth in these areas, forming solid stops

for the tray. On the side where the prepared tooth is situated, the stop should be distal to the

preparation. There should be a protective layer between the wax and the tray resin to prevent

the wax from impregnating the surface of the tray during the exothermic polymerization of

the acrylic resin. Adapt a piece of aluminum foil over the wax and stone cast to provide

separation.

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Mix the resin and form it into a rod that is approximately the length of the dental arch.

Place the acrylic resin on the arch and adapt it over the foil-covered wax. Place a horizontal

handle in the middle and a narrow ledge or "wing" on either side of it . The wings can be

used to get better leverage on the tray for removal from the mouth. Allow the resin to

polymerize.

If any grinding must be done on the tray, smooth and polish those areas that will

contact soft tissue before taking the tray to the mouth. The tray should be prepared at least 6

hours in advance. As the monomer undergoes polymerization, it shrinks as much as 8%

before curing is completed. Significant linear changes occur during the first 40 minutes of the

fabrication of a tray, with some changes continuing to occur for up to 6 hours. An acrylic

resin tray can be stabilized against further shrinkage and distortion by boiling it.

Paint the inside of the tray with a thin, uniform coat of adhesive, using the one

specified for the impression material being used. To achieve maximum adhesion of the

impression material to the tray, allow it to dry for a minimum of 15 minutes.

IMPRESSION MAKING :

Spatulation :

With polysulphide polymer , proper lengths of two pastes are squeezed onto the

mixing pad. The dark accelerator is first smoothed flat with a flexible yet stiff spatula and

incorporated into the white base. Holding the spatula flat against the pad, mixing is done

with a back-and-forth motion, pressing hard against the pad. Change directions often to

produce a smooth, homogenous mixture. The same procedure is followed in mixing the

polyethers and addition silicones .Spatulation time should be limited to 45 sec10.In case of

condensation silicone when the catalyst is supplied as a liquid, a specified number of drops

per unit length are mixed similar to that of two paste systems.

Methods of loading the syringe:

There are three methods for loading the syringe

1. Fold a sheet from the mixing pad in half and then fold it to make a cone. Open

it up and wipe the syringe material from the spatula onto it. Fold the cone

over. Squeeze the syringe material from the cone into the back end of the

syringe. Insert the plunger and express all the air from the syringe.

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2. In a second method of loading the syringe, the back end of the syringe is

brought in contact with the pad, and sweeped so that the material fills the

syringe .

3. In a third method, the syringe tip is removed and the front end of the syringe is

buried in a collected mass of material on the pad or in a dappen dish and

aspirated

Application of syringe and tray material:

Good access to the prepared teeth facilitates accurate placement of low viscosity

impression material. The teeth and the surrounding areas are redried with air. The retraction

cord is removed surrounding the teeth . the operator should immediately inject the light body

syringe material so that it allows the impression material to be deposited onto dry tooth

surface before any fluid can seep over the surface. This is important because the impression

material does not displace the fluid.

The tip should just be held above the mouth of the crevice without dragging the tip

along the gingiva. It should proceed smoothly around the entire circumference of the

preparation, pushing impression material ahead of the tip so that the entire tooth is covered.

Using an air syringe a stream of air is directed against the material to spread it evenly over

the surface of the preparation and to drive it into small details such as grooves and boxes.

Impression material is also forced more completely into the gingival crevice. The loaded tray

is then seated to allow the stops to orient the tray and limit its cervical movement.

Tray stabilization and removal

The impression tray must be held passively until the material attains sufficient body

so that it will not be distorted by tongue or cheek movements. After the material has set, it

should be removed in a direction parallel to the path of insertion. This can be accomplished

by grasping both buccal tray flanges, one with one hand and one with other hand, and

simultaneously exerting an occlusally directed force.

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The impression is rinsed to remove the blood and saliva. It is then dried and

inspected.

Disinfection

Microorganisms in the oral cavity can be transmitted from the impressions to the

dental laboratory. So the clinicial must disinfect the impressions before pouring the cast. An

alternative to disinfecting impressions is to use disinfected die stones. This approach avoids

potential distortions related to the procedures essential for disinfecting impressions.

There are five types of chemical disinfectants that can be used for this purpose:

chlorine compounds, combination synthetic phenolic compounds, glutaraldehydes,

iodophors, and phenolic/alcohol combinations.

The first step of any disinfecting technique is to rinse with water. This removes

significant amount of microorganisms. Disinfection technique involves sprayinf the

impressions with disinfection agents or immersion in agents like sodium hypochlorite.

Immersion in chemical agents should not exceed more than 30 min. Poly vinyl siloxane are

stable in this regard but special care should be taken with water based materials to avoid

distortion.

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OTHER IMPRESSION MATERIALS AND SPECIAL

CONDITIONS

Polyether Urethane Dimethacrylate

It is a polyether urethane dimethacrylate resin with a diketone initiator, an amine

accelerator, and 40% to 60% silica filler. It is used in a clear tray and is photoinitiated by

400- to 500-nm wavelength blue light, thus creating its most useful traits: a nearly unlimited

working time coupled with a short setting time12.

Impressions for Pin-Retained Restorations

To make an impression of a preparation for a pin-retained restoration, nylon bristles

must be used to duplicate the pin holes. Impression materials will not fill the small-diameter

holes being used. If a kit such as the V.I.P is being utilized, use the nylon bristle supplied

with a given diameter drill. It will be approximately 0.002 inch smaller in diameter than the

drill. If it is necessary, shorten the bristle to prevent it from hitting the impression tray and

being distorted.

Place a bristle in each of the pin holes. Proceed with the impression in the usual

manner, making sure to inject all the way around the head of the bristle. Withdraw the

impression in the line of draw of the preparation and pins.

Pour the impression in the usual way. When the stone has set, separate the impression and

cast. The nylon bristle used to duplicate the pin hole will remain in the cast. Remove it by

pulling it out with office pliers.

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